A voltage controlled oscillator (or “VCO”) is a type of electrical oscillator that generates an oscillatory signal having a frequency that is dependent on a control voltage. VCOs play an important role in communication systems and other electronic systems. They are used in both analog and digital applications, and are essential components of radio frequency communications systems.
FIG. 1 illustrates how a VCO is configured in a polar modulation transmitter 100. The polar modulation transmitter 100 comprises an amplitude modulator 102 in an amplitude modulation (AM) path, a VCO 104 in a phase modulation (PM) path, and a power amplifier (PA) 106. The amplitude modulator 102 functions to modulate a direct current power supply signal Vsupply according to amplitude variations in an amplitude modulation (AM) signal received in an AM path. The resulting amplitude modulated power supply signal is supplied to a power supply port of the PA 106. Meanwhile, a phase modulation (PM) signal received in the PM path is used as a tuning voltage, Vtune, which modulates the VCO 104 to generate a constant-amplitude phase modulated carrier signal. The PA 106 is typically implemented as a switched-mode PA operating in compression. Accordingly, as the constant-amplitude phase modulated carrier signal is amplified by the PA, the amplitude information contained in the amplitude modulated power supply signal is superimposed on the signal at the output of the PA 106.
Modulating the VCO 104 with the PM signal can be effective for some narrowband applications. However, as illustrated in FIG. 2, in wideband applications the VCO 104 is incapable of providing a linear response over the entire PM bandwidth. The nonlinear response results in distortion of the polar modulation transmitter output signal, making it difficult or impossible to comply with communications standards specifications. It would be desirable, therefore, to have methods and apparatus for avoiding or countering VCO nonlinearities in communication applications, including wideband polar modulation transmitter applications.